The invention relates generally to steam turbines and more specifically to exhaust hoods for steam turbines.
In the discharge of exhaust steam from an axial flow turbine, for example discharge of this exhaust steam to a condenser, it is desirable to provide as smooth a flow of steam as possible and to minimize energy losses from accumulation of vortices and turbulences and non-uniformity in such flow. Usually the exhaust from the turbine is directed into an exhaust hood and from there to through a discharge opening in the hood in a direction essentially normal to the axis of the turbine into a condenser. It is desirable to achieve a smooth transition from axial flow at the exhaust of the turbine to radial flow in the exhaust hood and thence a smooth flow at the discharge opening of this hood into the condenser.
In the constructing of an effective exhaust hood for use with such an axial flow turbine it is desirable to avoid acceleration losses within any guide means employed therein and to achieve a substantially uniform flow distribution at the discharge opening of the exhaust hood for the most efficient conversion of energy in the turbine and effective supplying of exhaust steam to the condenser to which it is connected.
It is also desirable to achieve optimum efficiency at the last stage buckets of the turbine prior to exhaust from the turbine by achieving a substantially uniform circumferential and radial pressure distribution in the exit plane of the last stage buckets. Finally, it is desirable to accomplish these results while employing a hood having as short an axial length as possible.
Prior art has employed, in the exhaust hood of the steam turbine, vanes having smoothly curved surfaces for changing the axial flow of the steam from the turbine to the generally radial flow. For example of such an arrangement for converting the axial flow of the exhaust from the turbine to radial flow is shown in U.S. Pat. No. 3,552,877 by Christ et al. Further developments in prior art exhaust hoods for axial flow turbines, such as U.S. Pat. No. 4,013,378 by Herzog, have incorporated multiple sets of vanes for further smoothing flow.
Such arrangements, however have not fully provided for effectively directing of the exhaust steam to the discharge opening of an exhaust hood with reduced acceleration losses and reduced losses resulting from the forming of energy-consuming vortices in the flow of the exhaust steam. Moreover, they have not fully achieved substantially uniform circumferential and radial pressure distribution at the exit plane of the last stage turbine buckets, a consideration which is of increasing importance for buckets having high tip speeds and high exit Mach numbers.
Diffusers are commonly employed in steam turbines. Effective diffusers can improve turbine efficiency and output. Unfortunately, the complicated flow patterns existing in such turbines as well as the design problems caused by space limitations make fully effective diffusers almost impossible to design. A frequent result is flow separation that fully or partially destroys the ability of the diffuser to raise the static pressure as the steam velocity is reduced by increasing the flow area. This is often caused by a vapor boundary layer that gets thicker along the diffuser surface in the direction of flow ultimately permitting the flow separation mentioned above.
U.S. Pat. No. 5,167,123 by Ronald E. Brandon describes a method and apparatus for improving the performance of vapor turbine diffusers by preventing flow separation from the diffuser walls. Such separation from the diffuser walls is decreased or eliminated herein by chilling the diffuser walls below the saturation temperature; causing some condensation to occur and insuring vapor flow toward the walls to eliminate the natural tendency toward separation in diffusing vapor passages.
Although using flow vanes may smooth the flow of steam from the last stage of the turbine to the condenser and the cooling of the diffuser walls may improve the performance of vapor turbine diffusers by preventing flow separation from the diffuser walls, other high velocity steam flow areas remain the exhaust hood. Accordingly, it may be desirable to provide further measures to reduce areas of high flow velocity within the exhaust hood.